1. Field of the Invention
The present invention relates to a process and apparatus for continuously producing an elastomeric composition. More particularly, the present invention relates to a process and apparatus for continuously producing an elastomeric composition by means of at least one extruder, the resulting elastomeric composition being primarily, but not exclusively, intended for use in the production of tyres.
2. Description of the Related Art
Conventionally, the production of elastomeric compositions (in the following also indicated as “rubber mixtures”) is performed batchwise by means of internal mixers, usually Banbury mixers having two counter-rotating rotors which exert an intensive mixing action to masticate the rubber ingredients and to incorporate and thoroughly disperse therein the other ingredients, including fillers, lubricating aids, curatives and auxiliary substances.
The compounding process using internal mixers shows many drawbacks, particularly a poor heat dissipation and thus a scarce temperature control, mainly due to an unfavourable ratio between material volume and mixer surface area. To improve dispersion in the rubber base, the various ingredients, and particularly the fillers, are incorporated into the rubber base in batches distributed in a plurality of mixing operations separated by cooling and stocking steps. Temperature sensitive ingredients, such as cross-linking agents and accelerators, are added only during the final mixing step after cooling the rubber mixture below a predetermined temperature (usually below 110° C.) to avoid scorching.
Therefore, the compounding process in internal mixers, although still remaining the most widely used mixing process in the rubber industry, is time and energy consuming and does not guarantee an effective control on the characteristics of the resulting elastomeric compositions, particularly as regards dispersion homogeneity of fillers into the rubber base. Variation in the added amounts of individual ingredients, timing of addition and discharge from the mixers, initial temperature of the raw materials, and fluctuations of shear forces inside the material during mixing, all contribute to batch-to-batch variation.
To overcome the limitations of the discontinuous processes, many attempts have been performed by the rubber industry to set up continuous compounding processes, based on extrusion techniques analogous to those commonly employed in the processing of thermoplastic polymer materials. Continuous mixing processes carried out by means of an extruder should improve uniformity in the rubber mixture characteristics, better thermal management resulting from improved surface-to-mass ratios, and possible development of highly automated operations. For an overview on this subject see the article “A tale of continuous development” by H. Ellwood, published in European Rubber Journal, March 1987, pages 26-28.
U.S. Pat. No. 4,897,236 discloses a process and an apparatus for continuously producing a rubber mixture, wherein the ingredients of the mixture are fed, masticated and homogenized in a twin-screw extruder. The resulting mixture is divided into a first and a second portion. The first portion is discharged, while the second portion is recycled for further homogenization and for mixing with fresh batches of the ingredients being fed into the extruder. The recycled portion is circulated to and returned from a cooled, annular chamber exterior to the extruder chamber, said annular chamber having outflow and inflow passages communicating with the interior of the extruder. That partial recycling of the rubber mixture should compensate for fluctuations in the metering of the ingredients and for local inhomogeneities which may occur. Moreover, the intensive cooling of the recycled portion in the annular chamber should correct a rising processing temperature, and should improve the dispersing action because of increased shearing stresses consequent to the temperature decrease.
U.S. Pat. No. 5,302,635 discloses a method and apparatus for continuously producing a rubber composition. In a first step, crude rubber is continuously fed into a twin-screw extruder, added with non-reactive additives (oils and fillers) and the resulting mixture is plasticated and homogenized by the extruder screws. During that first step, the mixture is kept at a temperature of from 100  C. to 160° C. Then, in a second step, the resulting mixture is cooled to a temperature of from 100° C. to 120° C. and reactive additives (particularly sulfur and vulcanization accelerators) are fed and incorporated into the rubber mixture. The homogenized end rubber composition then leaves the extruder via the extruder outlet opening.
The process can be carried out according to different extruder configurations. For instance, the two mixing steps can be performed in a single twin-screw extruder having two distinct mixing zones operating at two different temperatures. Alternatively, the first step may be carried out in a first twin-screw extruder operating at 100° C.-160° C.; the resulting base composition is then fed directly to a second twin-screw extruder operating at 100° C.-120° C. According to another embodiment, the process may be performed in a single extruder having two screw pairs driven at mutually opposite ends of the extruder housing, the two screw pairs operating at different temperatures.
U.S. Pat. No. 5,158,725 discloses a method for continuously producing elastomer compositions which comprises: feeding an elastomer into a twin-screw extruder; feeding at least one filler, oil and/or other elastomers into the extruder; mixing the ingredients to provide a homogeneous mixture which is maintained at a Mooney viscosity ML(1+4) at 100° C. between 20 and 250 during mixing; discharging the resulting mixture from the extruder. Precise volumetric or loss-in-weight feeders are used to meter the elastomer and other ingredients into the extruder. After leaving the extruder, the compounded rubber may be extruded through a die, calendered into sheets, strips or strands, or may be pelletized. The continuous method is less expensive than the multi-step batchwise processes currently used in the art and requires less manpower and less material handling. Moreover, improved dispersion and homogeneity of the resulting elastomeric compositions would result.
U.S. Pat. No. 5,262,111 discloses a process for the continuous production of a rubber composition in a twin-screw extruder. Rubber is fed into the extruder together with a processing aid and masticated up to a temperature of 120° C. to 180° C. Subsequently, a first part of carbon black, representing preferably 40-80% of the whole quantity of carbon black, is fed into the heated extrudate. Afterwards, plasticizing oil is added to the extrudate before the second remaining carbon black part is fed and incorporated into the extrudate at a temperature of from 120° C. to 180° C. The whole composition is then cooled to a temperature of from 100° C. to 120° C., a crosslinking agent is added, and the composition is homogenized and extruded. The process would improve the dispersion of carbon black in the extrudate while reducing the specific energy requirement.
U.S. Pat. No. 5,626,420 discloses a continuous mixing process and apparatus, wherein base elastomer(s) and other components are continuously dosed and introduced into a mixing chamber formed of a stator and a rotor rotating therein, preferably a single screw extruder. The introduced components advance within the mixing chamber along zones of propulsion and mixing. To improve dispersion and homogenizing of the rubber components, the filling rate of the mixing chamber in at least certain mixing zones is lower than 1. To properly introduce the components, and particularly the rubber base, into the mixing chamber, force feeding means are used, such as volumetric pumps (e.g. gear pumps). To obtain precise dosage of the different components, it may be desirable to add the components in a mixing zone where the filling rate is equal to 1, located between two mixing zones having a filling rate lower than 1.
U.S. Pat. No. 5,374,387 describes a process for continuously producing elastomeric compositions using a twin-screw extruder, which comprises the following sequential steps. In a first mixing zone of the extruder an elastomeric material is added, sheared and heated to a first operating temperature (typically from 130° C. to 220° C.) while reducing viscosity. Then the elastomeric material is passed in a second mixing zone where it is added with at least a first portion of a reinforcing filler and processing aid, while simultaneously cooling the rubber mixture to a second operating temperature (typically from 110° C. to 160° C.). The resulting mixture is then passed to an optional third mixing zone, where small constituent chemicals, such as adhesion promoters, anti-ozonants, color additives, fire retardants and the like, are introduced into the rubber mixture. Preferably, in said third mixing zone a second portion of the reinforcing filler and processing aid is added so as to reach a third operating temperature (typically from 85° C. to 130° C.). Then, in a fourth mixing zone the rubber mixture is supplemented with the vulcanization agent at a fourth operating temperature (typically from 115° C. to 150° C.). The mixture flow is then directed through a delivery zone (fifth zone) wherein the mixture flow is extruded into the desired form through some sort of die slot or the like. The various components of the rubber composition are continuously and individually metered to the extruder, preferably in the form of particulated materials and/or liquids by means of weight loss feeders.
U.S. Pat. No. 5,711,904 discloses a method for continuous mixing of elastomeric compositions reinforced with silica. A twin-screw extruder is fed with the elastomeric material, then with silica and other compounding ingredients, including a silica coupler. Temperature and pressure along the extruder are controlled to enable the silica coupler to react with the silica and the elastomeric material. Then curatives and accelerators are added, while maintaining the mixture at a Mooney viscosity ML(1+4) at 100° C. between 20 and 250. The mixing is continued for a time sufficient to thoroughly mix the curatives and accelerators. The resulting elastomeric composition is then forced through a suitable die mounted at the extruder exit. The overall process may be performed using a single extruder or a sequence of extruders. Preferably, residence time is increased in a first twin-screw extruder and then the composition is cooled, ground and dumped into a second twin-screw extruder where the rubber mix is completed with curatives and other ingredients. The different extruders may be separate independent entitites or may be coupled to each other to form one continuous process. The extruders may be closely coupled in a cross-head extruder mounting, or may be more loosely connected, for instance via festoons or belts that convey the material from one unit to the other.
In the Applicant's view, one of the most critical aspects in the production of an elastomeric composition by a continuous process is the addition to the rubber base of the so called “minor ingredients”, which are those components different from rubbers, reinforcing fillers and plasticizing agents (e.g. vulcanizing agents, vulcanization accelerators and retardants, protective agents, hardening resins, etc.), which are added to modify and/or to improve the characteristics of the elastomeric compositions. The minor ingredients are very numerous (usually at least 5-15 in a single rubber mixture) and used in little amounts (generally not greater than 5% by weight with respect to the total weight of the rubber mixture). Types and amounts of the minor ingredients to be added vary according to the elastomeric composition to be produced.
Usually, in the discontinuous compounding processes the minor ingredients are managed off-line. They are weighed as raw materials, charged into bags of a low melting material, sealed and automatically labelled with barcodes and alphanumeric data. Then, the labelled bags are manually introduced in the mixer hopper according to predetermined procedures which depend on the recipe of the rubber mixture to be produced. A tyre manufacturer typically has to manage about 30-40 different minor ingredients.
The above off-line managing of the minor ingredients is totally unfeasible in a continuous compounding process, where all the ingredients must be continuously and automatically dosed and fed into the extruder. The dosage shall be very accurate and the feeding shall proceed smoothly, since dramatic variations in the properties of the final rubber mixture may occur because of even little fluctuations in the added amounts of the minor ingredients.
Furthermore, addition of the minor ingredients as raw, substantially pure products shows many shortcomings. Firstly, the raw products are usually in the form of powders or of liquids, which are hardly dispersible in the rubber base and therefore require long mixing times. Additionally, an accurate continuous metering of little amounts of a concentrated product is difficult on an industrial scale, particularly when the product is poorly flowable, for instance in the case of low melting compounds or materials which show electrostatic charge accumulation. Finally, the raw products may give off vapours and/or powders which can cause unpleasant odours and safety problems, particularly in the case of noxious or toxic substances.